1. Field of the Invention
The present invention relates to a die bonding apparatus for bonding a semiconductor chip onto a chip substrate by using of a brazing filler metal.
2. Description of the Related Art
In the process of manufacturing a semiconductor integrated circuit such as an IC and a LSI, die bonding for bonding a semiconductor chip onto a chip substrate is performed, and to perform die bonding, a die bonding apparatus is used. A known die bonding apparatus has, for example, a structure shown in FIG. 9. The illustrated die bonding apparatus has a substrate holder 4 for holding a chip substrate 2. The substrate holder 4 comprises a positioning member 6 for positioning the chip substrate 2 in a die bonding work area, and a supporting member 8 for supporting the chip substrate 2 in the die bonding work area. The positioning member 6 comprises a block-form member, and a positioning concave 10 is provided in the center of the positioning member 6. When die bonding is performed, the chip substrate 2 is positioned in the positioning concave 10. The supporting member 8 comprises, for example, an oven having a flat top surface, and the chip substrate 2 is placed on the top surface. A heater 12 is incorporated in the supporting member 8.
The die bonding apparatus also has a collet 16 for conveying a semiconductor chip 14 onto the chip substrate 2 held in the die bonding work area. In the collet 16, an axially extending through hole 18 is formed. The through hole 18 is connected to a pressure-reducing source (not shown). When an action of reducing the pressure from the pressure-reducing source acts is on the semiconductor chip 14, the semiconductor chip 14 is sucked and supported by the tip of the collet 16. The collet 16 conveys the semiconductor chip 14 onto the chip substrate 2 in the die bonding work area while sucking and supporting the chip 14.
The semiconductor chip 14 is bonded onto the chip substrate 2 through a brazing filler metal 20. The brazing filler metal 20 is interposed between the chip substrate 2 and the semiconductor chip 14, and by heat-melting the brazing filler metal 20, the semiconductor chip 14 is bonded onto the chip substrate 2. Heat-melting of the brazing filler metal 20 is performed by the heater 12 incorporated in the supporting member 8. Lead wires 22 from the heater 12 are electrically connected to an AC power source 26 through a switch 24. Opening and closing, that is, the on and off of the switch 24 is controlled by controlling means 28. A temperature sensor 30 for detecting the heat-melting temperature of the brazing filler metal 20 is disposed in a predetermined position of the supporting member 8. The detection signal from the temperature sensor 30 is supplied to the controlling means 28.
In this die bonding apparatus, the chip substrate 2 is placed on the substrate holder 4 as required, and on the chip substrate 2 which is thus held is placed the semiconductor chip 14 by the collet 16 as required. When die bonding is performed, the collet 16 functions as pressing means, and presses the semiconductor chip 14 toward the chip substrate 2. When the chip substrate 2 is heated by the heater 12 under this condition, the brazing filler metal 20 is heat-melted by the heat from the chip substrate 2, and the brazing filler metal 20 is set, whereby the semiconductor chip 14 is bonded onto the chip substrate 2 as required. The temperature of the brazing filler metal 20 is controlled based on the detection signal from the temperature sensor 30. When the temperature detected by the temperature sensor 30 becomes lower than a predetermined lower limit temperature, the controlling means 28 closes (turns on) the switch 24, so that the heater 12 is heated to increase the temperature of the brazing filler metal 20. When the temperature detected by the temperature sensor 30 becomes higher than a predetermined upper limit temperature, the controlling means 28 opens (turns off) the switch 24, so that heating by the heater 12 is stopped to decrease the temperature of the brazing filler metal 20. By thus controlling the on and off of the heater 12 by the controlling means 28, the temperature of the brazing filler metal 20 is maintained within a predetermined temperature range of not less than the predetermined lower limit temperature and not more than the predetermined upper limit temperature, thereby ensuring bonding the semiconductor chip 14 onto the chip substrate 2 through the brazing filler metal 20.
However, in the conventional die bonding apparatus shown in FIG. 9, the following problems to be solved arise in association with the fact that the temperature sensor 30 is disposed in the supporting member 8 of the substrate holder 4. Since the chip substrate 2 is placed on the top surface of the supporting member 8 at each time of die bonding, when die boding is repetitively performed and the number of times of die bonding increases, the top surface becomes rough or dirt adheres to the top surface. When the top surface thus becomes rough or dirt adheres to the top surface, the adherence between the top surface of the supporting member 8 and the chip substrate 2 deteriorates, so that transmitting the heat from the heater 12 to the chip substrate 2 deteriorates. For this reason, there is a difference between the temperature detected by the temperature sensor 30 disposed in the supporting member 8 and the actual heat-melting temperature of the brazing filler metal 20, so that the controlling means 28 controls the temperature based on a determination that a desired heat-melting temperature is reached although the actual heat-melting temperature of the brazing filler metal 20 is lower than the desired temperature. Consequently, the brazing filler metal 20 is not sufficiently heat-melted, so that it is impossible to surely bond the semiconductor chip 14 onto the chip substrate 2.
Moreover, in this die bonding apparatus, since the side of the chip substrate 2 is heated, the temperature of the chip substrate 2 is held comparatively high. On the contrary, since the semiconductor chip 14 is pressed by the collet 16, the heat of the semiconductor chip 14 is absorbed by the collet 16, so that the temperature of the semiconductor chip 14 is lower than that of the chip substrate 2. Consequently, there is a temperature difference between the bottom surface of the chip substrate 2 that is in contact with the supporting member 8 and the top surface of the semiconductor chip 14 that is in contact with the collet 16. When the temperature difference is large, a thermal distortion occurs due to the difference in thermal expansion coefficient between the chip substrate 2 and the semiconductor chip 14, so that it is difficult to surely bond the substrate 2 and the chip 14.
Another prior art is shown in Japanese Unexamined Patent Publication JP-A-4-25137(1992). In this prior art, when a semiconductor chip is die-bonded onto a ceramic package, the semiconductor chip is gradually heated by a heater provided in a vacuum collet while the semiconductor chip being sucked and held by the vacuum collet is being conveyed to the package, and the semiconductor chip is heated substantially to the temperature of the package. By doing this, the semiconductor chip is prevented from being abruptly heated when the semiconductor chip is brought into contact with the package, thereby preventing the semiconductor chip from deteriorating by being abruptly heated.
Yet another prior art is shown in Japanese Unexamined Patent Publication JP-A-6-45377(1994). In this prior art, to curb the warpage of the semiconductor chip due to heat, a thermocouple and a heating resistor are provided in the collet body that sucks the semiconductor chip, and based on the temperature detected by the thermocouple, the collet body heated by the heating resistor is controlled so that the temperature of the collet body is held at a predetermined constant temperature, thereby curbing the warpage of the chip due to the heat at the time of die bonding.
Still another prior art is shown in Japanese Unexamined Utility Model Publication JP-U-63-20430(1988). In this prior art, a sheath heater and a temperature sensor are embedded in the collet. The temperature of the sheath heater is controlled based on temperature information from the temperature sensor, and after the semiconductor chip is vacuum-sucked by the collet, the semiconductor chip is pre-heated to an appropriate temperature before being mounted, thereby reducing the mounting time and relieving stress caused by an abrupt change in temperature of the semiconductor chip.
In the prior art shown in JP-A-4-25137(1992), a heater wire is provided in the collet that sucks the semiconductor chip and the collet is gradually heated. In the prior art shown in JP-A-6-45377(1994), the thermocouple and the heating resistor are provided in the collet body. In the prior art shown in JP-U-63-20430(1988), the sheath heater and the temperature sensor are provided inside the collet.
In the prior art, the semiconductor chip is heated by the heat from the collet, and a brazing filler metal or solder is heat-melted to perform die bonding. Since the temperature sensor is provided inside the collet to detect the temperature of the collet and does not detect the surface temperature of the collet, although the semiconductor chip is heated by the heat from the collet with the semiconductor chip being sucked by the end surface of the end portion of the collet, the heating temperature is controlled based on the inside temperature of the collet. For this reason, the adjustment of the heating temperature of the collet for heat-melting the solder or the brazing filler metal is inaccurate, so that there are cases where the solder or the brazing filler metal cannot be heat-melted to an appropriate state. As a result, the reliability of bonding of the semiconductor chip onto the chip substrate is low.